In a spread spectrum communication system, downlink transmissions from a base station to a mobile station include a pilot channel and a plurality of traffic channels. The pilot channel is decoded by all users. Each traffic channel is intended for decoding by a single user. Therefore, each traffic channel is encoded using a code known by both the base station and mobile station. The pilot channel is encoded using a code known by the base station and all mobile stations. Encoding the pilot and traffic channels spreads the spectrum of transmissions in the system.
One example of a spread spectrum communication system is a cellular radiotelephone system according to Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) Interim Standard IS-95, "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System" ("IS-95"). Individual users in the system use the same frequency but are distinguishable from each other through the use of individual spreading codes. Other spread spectrum systems include radiotelephone systems operating at 1900 MHz, commonly referred to as DCS1900. Other radio and radiotelephone systems use spread spectrum techniques as well.
IS-95 is an example of a direct sequence code division multiple access (DS-CDMA) communication system. In a DS-CDMA system, transmissions are spread by a pseudorandom noise (PN) code. Data is spread by chips, where the chip is the spread spectrum minimal-duration keying element. A key system parameter is the chip duration or chip time. In an IS-95 system, the chip clock rate is 1.2288 Mega-chips per second, equivalent to a chip time of about 0.814 .mu.sec/chip.
Mobile stations for use in spread spectrum communication systems commonly employ RAKE receivers. A RAKE receiver includes two or more receiver fingers which independently receive radio frequency (RF) signals. Each finger estimates channel gain and phase and demodulates the RF signals to produce traffic symbols. The traffic symbols of the receiver fingers are combined in a symbol combiner to produce a received signal.
A RAKE receiver is used in spread spectrum communication systems to combine multipath rays and thereby exploit channel diversity. Multipath rays include line of sight rays received directly from the transmitter and rays reflected from objects and terrain. The multipath rays received at the receiver are separated in time. The time separation or time difference is typically on the order of several chip times. By combining the separate RAKE finger outputs, the RAKE receiver achieves path diversity.
Generally, the RAKE receiver fingers are assigned to the strongest set of multipath rays. That is, the receiver locates local maxima of the received signal. A first finger is assigned to receive the strongest signal, a second finger is assigned to receive the next strongest signal, and so on. As received signal strength changes, due to fading and other causes, the finger assignments are changed. After finger assignment, the time locations of the maxima change slowly, and these locations are tracked by time tracking circuits in each assigned finger. If the multipath rays are separated from each other by at least one chip time of delay, then each path can be resolved separately by the RAKE receiver time tracking circuitry and diversity gain is realized.
On many channels, the multipath rays are separated by intervals of much less than one chip time. Current systems, however, lack the ability to resolve or separate multipath separated by such small intervals, for several reasons. First, if the channel is static and the multipath profile yields only a single local maximum when two closely spaced rays are present, the time tracking circuits of fingers assigned within one chip time of the local maximum will drive those fingers to the time location of the local maximum, and the benefit of channel diversity will be lost. Second, fingers may track to the same time location even if separated by a chip or more. If one path is strong while another path is in a deep fade, the delay-locked loop of the finger assigned to the faded path will detect sidelobe energy of the unfaded path and track to the unfaded path's location. Again, the fingers converge in time and diversity benefits are lost.
Accordingly, there is a need in the art for an improved RAKE receiver and finger management method which can realize the benefits of path diversity when multipath rays are spread by less than one chip time.